Colorado is a mountain biker's paradise. One of my favorite places to go is around Winter Park. There are lots of maintained singletrack trails there, both on and off the ski resort.
Another good place is around Salida. There used to be a gas station west of town that would haul bikers up to the top of Monarch pass in an old van for like 8 bucks. They would dump you out on top, where you would climb south towards the old Marshall pass, and from there you had several choices to get back down to town. It's definately not an all downhill ride though. In fact, you will dread seeing the term "regain the crest" on the trail map. It's anything from a very cool half-day ride to an all-day mondo downhill brutal crest-gaining trek back to town. Breathtaking scenery too.
Another good one is around Crested Butte. Though not quite as civil an atmosphere as around Winter Park there are more miles of single track here than you can ride in a single summer. Maybe two summers. And if you go here you MUST go to town and sit on main street one evening on one of their benches and watch the locals. I think they put up these benches just so out-of-town scuzzy mtn bikers like me can sit and giggle and laugh at those people. And to secretly admire their laid-back lifestyle.
I have more if you want, but writing this makes me hate this Oklahoma heat even more. I also know some people who own a condo in Crested Butte and they make killer deals for bikers in the summer months...